7 Answers

Do you need to measure it, or just filter it? Many substances will filter light with certain properties (generally, its wavelength or polarity). You could also measure by blocking some portion of the light, and/or measure and reproduce what you measure.

You can use a splitter to pass half the light the way you want and use the other half for measurement. I use Edmund Optics If they don’t have what you need, it does not exist.
It’s also fun to just browse the catalog.
Enjoy.

Yeah, but that would destroy my precious light.
It might be useful to mention this light source is not static and this measurement has to be integrated. I suppose I could settle for a way to measure the light souce efficiency, and then just use that in combination with the power I’m putting into it.

@Zyxthe light wouldn’t be destroyed, only converted into a different form of energy and/or re-radiated at different wavelengths

I don’t think it’s possible to measure the intensity of light directly without absorbing some of it. You can certainly calculate its theoretical brightness based on the specification of the components you use and the energy you put into the system, but that’s no guarantee that the calculation will accurately reflect the reality of the system.